Sensors comprising micro-electrode arrays are used for measuring electrical activity in small networks of biological cells, such as neurons. Theses sensors are often relatively big and only small matrices or arrays of micro-electrodes can be made. Current state-of-the-art micro-electrode arrays (MEAs) may contain a maximum of 64 electrodes with a minimal spacing of 100 μm between neighbouring electrodes. The electrodes are often made of flat TiN pads with a diameter of maximum 10 μm. For some applications, smaller spacings, e.g. <10 μm, and a larger number of electrodes, e.g. >60000, may be required.
On-chip single cell recording of electrical activity using field-effect transistors has been demonstrated for large neurons or tissue slices (see P. Bergveld et al, IEEE Transactions on Biomedical Engineering, 1976. P. Fromherz et al., Science, May 1991, A. Cohen et al, Biosensors and Electronics, January 2004). In the case of mammalian neurons, e.g. hippocampal neurons, the cells are much smaller, which leads to a less efficient electrical coupling onto the chip surface and make a reliable electrical contact between the cell membrane and the recording device.
Hoogerwerf et al. have disclosed an array of microneedles for sensing of neurons. The array of microneedles or probes is assembled to a silicon platform with a construction in which the microneedles run through holes in the platform and then are connected using a certain flange. Electrical connection is made with bumps, e.g. eutectic metal bumps or plated contacts on a plated gold beam. The beams are coupled to interconnects extending on the substrate flange and from there extend on the microneedles. Such needles are particularly suitable for probing a cell. The overall array with microneedles bears a large similarity to probe cards with a plurality of probes used for testing integrated circuits. The conductive probes of such an array make contact with selective test pads on the integrated circuits and provide series of suitably digital signals to the integrated circuit for testing proper operation. However, when applying such array of probes to a biological cell, an adequate measuring of cell properties turns out difficult. The cell membrane does not have a solid and fixed plate of test pads. Therefore, an adequate stimulation by electric pulses and subsequent measuring is not achievable.
Fujimoto et al. disclose “Electroporation microarray for parallel transfer of small interfering RNA into mammalian cells” in Analytical and Bioanalytical Chemistry, Springer, Heidelberg, Del., Vol. 392, no. 7-8, 12-2008, pages 1309-1316. The document recites transfer of chemical substances into living cells, specifically transfer of siRNA into mammalian cells by an electroporation microarray.
Braeken et al. disclose “Local electrical stimulation of single adherent cells using three-dimensional electrode arrays with small interelectrode distances” in Engineering in Medicine and Biology Society 2009, Annual Int. Conf. IEEE, IEEE, Piscataway, N.J., USA, Vol. 1, 3 Sep. 2009, pages 2756-2759. The document recites cell electroporation by applying voltage pulses by a microneedle being smaller than the cell.
Han et al. disclose “A single cell multi-analysis system for electrophysiological studies” in Transducers '03, 12th Int. Conf. Solid State sensors, actuators and Microsystems, 2003, IEEE, Piscataway, N.J., USA, Vol. 1, 9 Jun. 2003, pages 674-677. The document recites a micro analysis system for multi purpose electrophysiological studies of single cells. It recites an electrode at a sidewall for coupling between a cell and an electrode.
Huys et al. disclose “Novel concepts for improved communication between nerve cells and silicon electronic devices” in Solid State Electronics, Elsevier Science Publishers, Barking, GB, Vol. 52, no. 4, 3 Dec. 2007, pages 533-539. The document recites a high density matrix of sensors and actuators on a CMOS chip. The document recites that electrical coupling can be improved by increasing the contact area between cell and electrode.
Jaber et al. disclose “Action potential recording from dielectrophoretically positioned neurons inside micro-wells of a planar microelectrode array” in J. Neuroscience Methods, Elsevier Science Publishers, Amsterdam, NL, Vol. 182, no. 2, 15 Sep. 2009, pages 225-235. The document recites 4×4 planar microelectrode arrays for studying and organising in vitro neural networks at a cellular level. The document recites growth of a biological cell into a channel, and sensing and actuating a portion of the cell.
EP-A 1967581 discloses CMOS compatible microneedles. A diagrammatical cross-sectional drawing hereof is shown in FIG. 9. The microneedles 6 are present on a substrate 1. The compatibility with CMOS is enabled, in one embodiment, with a manufacturing process as typically in use for the manufacture of interconnect structures of integrated circuits. While Hoogerwerf et al provide contacts on the outside of the microneedle, EP1967581 defines conductors 3 and/or microfluidic channels inside the microneedle 6. Thereto a dielectric matrix with a conductor 3 on top of an interconnect 2 is defined. Subsequently, the dielectric matrix is patterned so as to define the microneedles 6 with an insulating shaft around the conductor.
The use of such CMOS compatible technology for manufacturing results therein that a top surface of the microneedle may be planar. This is particularly the case at a stage during processing of the microneedles, before the partial removal of the dielectric matrix around the microneedles. As a result, a sensor or actuator device 4 may be defined on top of the microneedle 6. The interconnects 3 and/or microfluidic channels then run towards the sensor or actuator device 4 on top of the microneedle 6. On application of a biological cell 40, engulfment of the microneedle 6 has been found. This engulfment is beneficial as it leads to reduction of the amplitude of a stimulation current for the actuation of the biological cell.
However, it turns out that the area of the sensor or actuator device on top of the microneedle is limited. Increase of the area by increasing the diameter of the microneedle turns out limited, as the engulfment of the microneedle 6 by the biological cell 40 may be lost, which is undesired.